Conventionally, a member that is capable of transmitting an electromagnetic wave such as a millimeter wave therethrough while having a metal-like outer appearance is devised as a casing component of an electronic apparatus or the like. For example, Patent Literature 1 discloses an exterior component for loading an automobile radar on an emblem of an automobile. For example, indium is deposited on a resin film and this film is attached to the surface layer of the emblem by an insert molding method. With this, it is possible to produce an exterior component that has an ornamentally metallic luster and has no absorption region in an electromagnetic frequency band due to the island structure of indium (paragraph 0006 of Patent Literature 1, etc.).
However, in the method of forming the island structure of indium, there is a problem that it is difficult to make a uniform film thickness on the whole in the case where the deposition area is large, for example. Further, there is also a problem that the island structure is easily broken due to the temperature of resin to be poured thereinto when forming the casing component (paragraphs 0007 and 0008 of Patent Literature 1, etc.).
In order to solve this problem, in Patent Literature 1, the following technology is disclosed. That is, a sea-island structure in which a metal region is regarded as an island and a non-metal region surrounding the island is regarded as a sea is artificially formed to have regularity. Then, each metal region is insulated with each other by the non-metal region, and the area of the metal region and the distance with an adjacent metal region are properly controlled. With this, a material that has electromagnetic wave transmission properties comparable to those of a film on which indium is deposited can be obtained (paragraph 0013 of Patent Literature 1, etc.).